1. Technical Field
The present invention relates to BiCMOS structures.
2. Description of the Related Art
BiCMOS structures consist of at least two MOS transistors and one bipolar transistor (BJT). Their inception has come about in an attempt to take advantage of the beneficial aspects of MOS and bipolar devices and their use is becoming increasingly prevalent.
Beneficial aspects of MOS devices include that MOS processing techniques yield high packing densities and MOS devices generally operate on less current than functionally analogous bipolar devices. A limitation of MOS devices is that they have poor current driving capabilities.
Beneficial aspects of bipolar devices include that while being more difficult to isolate and taking up more space than analogous MOS devices, they provide good current driving capabilities and attain higher operating frequencies than MOS transistors. Certain bipolar transistor structures such as a vertical BJT are capable of operating, at present standards, at approximately 80 Gigahertz (GHz). Laterally arranged bipolar transistors operate at approximately 10 GHz.
BiCMOS structures are currently used in SRAM and DRAM chips. In such circuits, MOS transistors are used to form memory cells and bipolar transistors are used for driving signals onto buses and other high capacitance loads. BiCMOS structures are also used in logic arrays for related purposes.
Another application of BiCMOS structures is in circuits such as A/D and D/A converters. In these circuits, both transistor types are integrated to produce a more accurate and more rapid conversion. The developing field of digital signal processing is a growing field of BiCMOS implementation.
Referring now to performance considerations, the performance of MOS transistors is impeded by parasitics, such as parasitic capacitance and parasitic resistance, that combine to form a time constant (.tau.) which reduces the maximum operating frequency of an MOS transistor.
With respect to bipolar transistor (which as stated above may be arranged either laterally or vertically), laterally arranged bipolar transistor have a large series resistance due to the thin silicon film from which they are made. This large series resistance adversely impacts operating frequency. By comparison, vertically arranged bipolar transistors have operating frequencies approximately one order of magnitude greater than that of laterally arranged bipolar transistor and, therefore, their use is generally preferred.
The present invention is directed towards enhanced performance in BiCMOS structures.